Optical transmission systems are widely used to transmit data on a broadband network. In a typical optical transmission system, a laser provides an optical signal at a predetermined frequency which is typically modulated to provide an optical transmission data signal.
In Broadband Passive Optical Network (B-PON) applications, a 1550 nm optical signal is assigned for video signal transmission. An externally modulated laser transmitter is typically used for optical signals because an externally modulated laser transmitter has much lower chirp than a directly modulated laser transmitter. A directly modulated laser transmitter, especially a high power laser transmitter, may have a total laser chirp up to several GHz due to the large laser modulation RF current. The large total laser chirp helps to improve the Stimulated Brillouin Scattering (SBS) suppression optical power level, but when combined with the fiber dispersion, serious second order distortions occur, such as Composite Second Order distortions (CSO).
When the chirped optical frequencies pass through the fiber with dispersion, different frequencies travel through the fiber with different group velocities, which introduces a delay and often causes distortions in the communication signal. CSO distortions often occur in the low −40 dBc range. For the B-PON laser transmitter to be useful, the laser transmitter CSO distortions should be better than −60 dBc. Accordingly, a high degree of CSO correction ability in the high frequencies and very accurate adjustable distortion compensator is needed in order to use a directly modulated laser.
If CSO distortion problem can be solved, a directly modulated laser transmitter has advantages in the B-PON system. In the directly modulated laser transmitter, the OMI is usually at least 1-2 dB higher than the externally modulated laser transmitter. The carry to noise ratio of the B-PON system using directly modulated laser transmitter can be 1-2 dB higher. Using the directly modulated laser transmitter, due to the large laser total chirp, the SBS suppression optical power level can be higher than the SBS threshold suppression optical power level for the externally modulated laser transmitter. This is very useful for B-PON applications. Besides, directly modulated laser transmitters are much cheaper than externally modulated laser transmitter. The reliability and temperature stability of the directly modulated laser transmitters are much better than the externally modulated laser transmitters.
For the B-PON applications, the longest distance is 20 km. So the dispersion compensation for B-PON application needs only to be compensated for up to 20 km fiber distance.
The problem of fiber dispersion compensation has been investigated in great detail and various techniques have been used to solve this problem. The solutions were in both optical domain and electronic domain.
In the optical domain, dispersion compensation fiber (DCF) or chirp fiber Bragg grating (CFBG) can be used for the compensation devices. DCF is an optical fiber that has the exactly the opposite dispersion effect as a regular single mode fiber. A CFBG is a component that reverses the group delay comparing to the ordinary fiber group delay between wavelengths.
The advantage of optical technique is its accuracy. The disadvantages are that the DCF is costly, adds attenuation, and needs additional amplifications and difficult to be readjusted. For the CFBG, the optical attenuation is low. The optical bandwidth is limited to about one nm. It makes the laser source choice more difficult and the laser wavelength needs to be stabilized.
While optical solutions are more expensive, the electronic compensation techniques benefit from being cost effective. In the electronic domain, the prechirp technique has been widely used in digital applications.
One method for the CATV application is described in U.S. Pat. No. 5,115,440 to Hermann Gysel, et al. In this patent, an electrical controlled varactor delay line network is inserted between a source of the laser modulating signal and the laser. The delay line network provides an instant amplitude dependent delay of the modulating signal applied to the laser and compensates the fiber delay caused distortion so that the CSO distortion can be reduced. One of the advantages of using this electrical compensation is that it is not sensitive to the optical wavelength and the compensation can be easily adjusted electronically. This approach worked very well for low optical power (1-3 mw) lasers with relative large laser chirp (1.8 Ghz/ma). However, modern 1550 lasers have much lower laser chirp and may have much large optical power, e.g., laser chirp now may be between 30-70 Mhz/ma and optical power may up to 10-13 dBm.
Large optical power is important for B-PON applications. Large optical output power laser usually has more laser chirp, so the SBS suppression optical power level will be larger. Further, when larger optical power goes to the eridium doped fiber amplifier (EDFA), it increases the carry to noise ratio of the systems.
The large optical power usually needs larger RF drive voltages. For example, for most 1550 laser power range from 10-13 dBm, the peak RF drive voltage will be around 4-8 volts. Comparing to the low optical power lasers, which RF driving voltages are less than one volt. Driving the varactor with large RF voltages, the capacitor change with voltage is nonlinear. Due to the large RF driving voltage, comparing to the prior art, the RF driving voltage needs to be predistorted in order to get linear change capacitor. Also, a very smooth control of dispersion compensation method is needed for CSO correction.